Chap. 11.] 
COURSE OF THE SAP. 
73 
Dr. Lindley thus confidently and complacently 
concludes his statement of the theory of Darwin 
and Du Petit-Thouars: — “ The elongation of 
the leaf-bud upwards gives rise to new axes with 
their appendages; their elongation downwards 
increases the diameter of that part of the axis 
which pre-existed, and produces roots.” 
The argument from the grafted stock is, I 
think, stronger against this theory than against 
the growth from the descending sap ; for though 
the Doctor may adopt the idea of the lateral 
flow of sap, and make it necessary to irrigate 
the roots of the buds, it would scarcely have the 
power to metamorphose an actual growing fibre 
of peach-wood into a fibre of plum-wood. 
Dutrochet and Link bring us back to Hales’s 
doctrine of the all-importance of the pith. In¬ 
deed, Dutrochet would establish the omnipresence 
of the pith. He tells us that each division be¬ 
tween the annual layers of wood is a pith for 
the layer outside it (let us call these concen- 
trical piths, to distinguish them from the central 
pith); and that, in addition to the original me¬ 
dullary rays, or silver grain, which run from the 
central pith or medulla to the bark, and which 
are annually prolonged through each successive 
concentrical pith and layer of wood, — in addition 
The pith or 
medulla, and 
medullary rays 
or silver grain. 
